Frequency selective circuits



May 17, 1938. c, w, GREEN 2,117,843

FREQUENCY SELECTIVE cmcums Filed Deg. 2, 1936 I l /5 n- /8 l /0 SIGNAL /4 6\ v RECEIVER rnmsl3 spa-cu J u/rrn l2 RECEIVER SILVER f SULPHIDE 5 a ass/sum: 7

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sum/ms RES/STANCE lNl ENTQR C. M. GREEN ATTORNEY am Mam, 193s UNITED; STA ES PATENT OFFICE- FREQUENCY SELECTIVE CIRCUITS Charles W. Green, MillburnfN rl, asaignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation. of New York Application December 2, 193a, Serial No. 113,820

1 Claims. (Cl. 173-84) This invention relates to frequency selective circuits such as may be employed in discriminating between signaling currents of diflerent frequencies.

One object of the invention is to increase the impedance of a tuned circuit to frequencies differing materially from the frequency to which the circuit is tuned while offering a substantially reduced impedance to current of the critical frequ cv- I Another object is to maintain a frequency selec-- tive circuit of high impedance to the frequency to which it is resonant, unless the current of said frequency is applied longer than a definite time interval. v

A further objectis' to provide a resonant circuit, the characteristics of which depend upon the amplitude as well as the frequency of the applied voltage.

The invention is principally diversion from a transmission linefof a current of a single frequency or narrow band of frequencies. For example, in'voice frequency ringing over a telephone line, it is desirable to divert the voice frequency signal into its proper receiving circuit without introducing appreciable loss in the other frequency bands transmitted over the line. In one embodiment, the invention may comprise inshunt to a transmission line a condenser,

30 a resistance element such as a silver sulphide resistance having a high negative temperature eihcient of resistance, and the primary winding of a transformer, the transformer and the condenser being resonant at the frequency to be diverted,'for example, a ringing current of 1,000

cycles per second. The characteristics-01a silver sulphide resistor are such that it presents a high resistance when subjected to voltage below a cer- .tain critical value but as the voltage is increased, the temperature of the resistance is increased until a critical point is reached at which the resistance drops to a very low value. The above described bridge circuit because of the impedance of the tuned circuit and because of the high resistance of the silver sulphide element for thev usual low voltages of speech currents near a receivingstation introduces a negligible loss at all frequencies at these low voltages.

/ However, when the signaling frequency to which the bridge is tuned is on the line in suiiicient amplitude and for suiiicient' duration of time the resistance of the. silver sulphide element will suddenly break down'to a very low value to enable the tuned circuit to absorb most of the signaling frequency. The receiving apparatus for adapted to the Further details of a system of this type may be secured by reference to the H. S. Black United States Patent 1,709,554, issued April 16, 1929, on

Voice frequency calling system.

The transmission line 5 may be traversed by speech frequencies or by a carrier wave modulated by the voice from a source such as source 6 to be received at a distant station by suitable receiver I which for carrier operation would include a suitable demodulator or detecting device. As

is customary in such systems it will also be de* sirable to transmit over the line a voice frequency wave to actuate calling or signaling apparatus at the distant station. In shunt to line 5 at one station is a source 8 of signaling current and a key 9 for connecting the source to the line. ,Source I. for example,-may comprise a 1,000 cycle generator interrupted twenty times per second.

At the distant station there is connected in shunt to the line the primary winding ll of a transformer, a condenser ,and a silver sulphide .resistor l2. Winding II and condenser ii are tuned for series resonance at the frequency of the source I. The secondary winding 'II of the transformer leads to a suitable signal receiver ll.

For the low voltages of the speech or modulated carrier at' the receiving station, the silver sul-- phideresistor i2 possesses a very high resistance and the shunt circuit ill, i2, therefore, introduces a negligible loss for all frequencies in the speech 40 or carrier bands provided the voltage across the silver sulphide resistor is below its critical value. When, however, the higher voltage from the sig-' naling source 8 is impressed on the line, there is an enormous reduction in the value of resistance I! so that the line is substantially short-circuited. It follows, therefore, that the bridge circuit II, II receives the signaling frequency from source I with full eiiiciency.

If the silver sulphide resistor i! were absentfrom the bridge circuit ll, I9 the tuned circuit II, II would be'required to put in a loss as high as 18 decibels to the signal frequency of source in order that the bridge would have a negligible loss at speech frequencies of 0.2 decibel or less. #3

This high loss in such a bridge circuit would require amplification of-the signaling frequency from source 8 before the actuation of the signal receiver I 4. However, when the silver sulphide resistor is present, the-bridge circuit not only has a negligible loss for speech frequencies but also 1 has a very low loss at the frequency of the 318' naling source 8.

The silver sulphide unit employed in the above described tuned circuit should preferably be of such size and, thermal design that its temperature due to the received voltage from frequency source 8 does not reach the critical value-unless this voltage persists for an appreciable time such as 0.5 second. This will aid the signal receiver ll in discriminating against momentarily im-' and the tuning condenser may be placed in series with the secondary winding [3 instead of in series with the primary. winding II. This modification is shown him. 2 where a tuning condenser ll and a silver sulphide resistance ll are connected in series with the secondary winding If. The transmission characteristics of the bridge circuit i8, I! 01' Fig. 2 are the same as those described above for Fig. 1.

For various methods which may be'employed in making a suitable silver sulpbide resistor, referary 2, 19 36,

ence may be made to the following copending applications: J. R. Fisher application Serial No. 30,780, filed July 11, 1935; J. R. Fisher application Serial No. 18,217, filedApril 25, 1935; and R. 0..

Grisdale application Serial No. 57,180, filed Janu This inclusion in coeflicient of resistance may be advantageously employed in widely difierent types of frequency selective circuits to obtain a-selectivity which depends not only upon the frequency but the voltage eflicientof resistance may of the silver sulphide element.

' transformer and a resistance element having ,a a high negative temperature coeiiicient of resistance, said path being tuned to a. frequency of said' as well.

It is also to be understoo that other substances having a high negative temperature co- What is claimed is: r

1. In a signal transmimion system, a sourceof signaling current of a wide band offrequencies a second source of signaling current of a narrow band of frequencies, a line, means for impressing on said linesignaling currents from both of said s sources and a path bridged across said line comprising a condenser, the primary winding of a second source, said path being of high impedance to the signaling current from said first 2. In a traon system, a line, a source of speech currents, a source of ringing current of a definite frequency, means for impressing currents from SMLSOHICGS upon said line, a receiving station whcze the current received over said line from said second source is of higher voltage than be emnloyed inplace 1 tthe-current received from said first source, and means at said receiving station for receiving current from said second source, said means comprising in shunt to said line the primary winding of a transformer and a resistance element having a high negative temperature coefiicient of resistance.

3; In a signal on system, a line, a source of signaling currents of a wide band of frequencies, a source of ringing current of a definite-frequency, means for impr currents from said sources upon said line, a receiving sta-.

tion where the current received from said second the transmission of the low voltage speech currents thereover," said means including a nonlinear resistance, an inductance connected across said circuit in series with said resistance, and means responsive to the ringing current potential developed across the terminals of said inductance. 5.'In a telephone system, a telephone circuit designed for 'the'on of low voltage .sp'eech currents, and a ringing circuit associated therewith comprising a high voltage ringing'current generator, a silver sulphide resistor which presents a high impedance to the low voltage speech currents and a. relatively low impedance at the higher voltage of said generator, an inductance in series with said resistor, andmeans responsiveto' the rlnging current; potential de- 7 V q I veloped across the terminals ofsaid inductance. a'resonant circuit of a resist- 7- ance element having a high negative temperature 6'. inasignallng n em. asourceofsignaling' current of a wide'band of frequencies, a second of signaling current of a narrow band of frequencies, a line, means for impressing on said line signaling-currents from both of said sources, a-receiving station where current received over I said line from said secondsom'ce is of higher voltage than the current received from said first source, and means at said station for providing: in shlmtto said line a path of high impedance 7 transformer, .a condenser, a element having a high negative temperature coefficient of resistance, anda signal receiver for the currents from said second source, saidseries connection being tuned to afrequency of said second 1 r 01mm W. QREEIL- 

